With the prospective emergence of the All Electric Airplane, in which all functions and services are performed electrically, the flight-control surfaces will be activated by some type of electromechanical actuator system (EMAS). Typically, such an EMAS will interface with a quad-redundant fly-by-wire/fly-by-light (FBW/FBL) system that will furnish command information to the EMA, with flight control system computers being in turn responsible for the control and management of the EMAS and the airplane's flight characteristics.
In the past, and in most current large airplanes, the control surface actuator function is performed with hydraulic jacks or cylinders. In the event of a hydraulic supply failure (or a failure in the closed loop servo control or a stuck servo-valve etc.) in such systems the hydraulic ram can be bypassed to permit other actuators, connected to the same surface, to continue operation. In the case of the EMAS, however, there is concern that a broken-tooth in a gear-train, or a bearing seizure will inhibit the operation of other (redundant) actuators, for there is no simple method to bypass or free-wheel, the jammed actuators.
Prior examples of attempts to solve this problem include U.S. Pat. No. 2,441,247 "Mechanical Translating Device" by A. W. Mooney which discloses an electrically powered actuator rotatably mounted to the aircraft structure. Rotation of the actuator is prevented by a circular gear mounted to the actuator which is in engagement with a worm gear. Thus, in normal operation, rotation of an output shaft can be accomplished because the worm gear prevents the actuator from rotating. Should the actuator fail, the worm gear can be rotated by mechanical means or any other drive means, thus still providing an effective output.
The problem with this actuation system is that if redundancy is required, i.e., two separate actuators controlling the same control surface, there is no means to automatically disengage a failed actuator, since the worm gear cannot be back driven, effectively preventing the actuator from rotating.
Another patent of interest is U.S. Pat. No. 3,950,686 "Series Reedundant Drive System" by James C. Randall. Here a plurality of motors are mechanically connected in series so as to provide operational redundancy. The shaft of a given motor is rigidly attached to the housing of the next motor in the series so that rotation of the first shaft will cause the motor and its shaft to rotate. While this system will provide effective operation should one motor fail, the output shafts are held in position by means of the magnetic attraction between the rotor and poles. This does not provide for a positive lock on the position of the control surface connected thereto.
Still another patent of interest is U.S. Pat. No. 4,289,996 "Actuators" by R. R. Barnes et al. Here a pair of motors are coupled to a differential gear assembly which in turn drives a screw assembly attached to the load (a control surface). A lock mechanism is provided on each motor which will lock the motor and prevent its output shaft from rotating should failure of the motor occur. This allows the other motor to continue to drive the control surface through the differential gear assembly. This system requires that each motor be coupled together by a differential gear assembly. Should the differential gear assembly fail, both motors become ineffective.
Other patents of interest are U.S. Pat. No. 3,790,108 "Redundant Stabilizer Control" by J. W. Bock; U.S. Pat. No. 2,315,110 "Control Apparatus for Aircraft" by C. Dornier; U.S. Pat. No. 2,491,842 "Actuator System" by B. A. Wells; U.S. Pat. No. 2,549,815 "Servo Unit" by W. L. Huntington; U.S. Pat. No. 3,140,843 "Servo System" by R. H. Pettet; and U.K. Pat. No. 576,797 "Improvements in Electric Motor Drive Units" by C. Heal.
Therefore, it is a primary object of the subject invention to provide an actuation system for controlling the position of a control surface of an aircraft wherein a jammed or disabled actuator can be effectively bypassed and, thus, allowing other actuators to freely function.
It is another object of this invention to provide an actuation system having a simplified unlocking means for a jammed or disabled actuator by allowing the actuator to freely rotate about its axis of rotation while remaining connected to the control surface.
Another object of the subject invention is to provide an actuation system wherein a primary actuator is coupled with a secondary or emergency actuator and a simplified means is provided to free a jammed primary actuator so that the secondary actuator can actuate the control surface by rotating the primary actuator.